Many structures require two or more plates that are held parallel to one another with a precise separation between them. Examples include liquid crystal displays, plasma displays, and optically resonant cavities, such as a Fabry-Perot interferometer or Fabry-Perot etalon. An optically resonant cavity is a well-known structure that is defined by two partially reflective parallel surfaces that are separated by a precise distance. This distance is referred to as the cavity length. For light of a particular wavelength, the reflectivity and transmissivity of the optically resonant cavity are functions of the cavity length. A Fabry-Perot etalon has a fixed cavity length, while a Fabry-Perot interferometer has a cavity length that can vary.
A Fabry-Perot interferometer is used as the basis of many optical displacement sensors, wherein its cavity length varies in response to an environmental stimulus, such as acceleration, vibration, pressure, temperature, sound, etc. In some of these sensors, one of the surfaces of the optically resonant cavity is a surface of a membrane that moves in response to the environmental stimulus. When the movable membrane moves in response to the environmental stimulus, the reflectivity and transmissivity of the Fabry-Perot interferometer changes. Photodetectors detect the light reflected and/or transmitted by the Fabry-Perot interferometer and generate electrical signal(s) based on the intensity of the detected light. An electrical signal based on the environmental stimulus is thereby generated.
The optical performance of a Fabry-Perot interferometer-based sensor can be highly dependent upon the initial separation (i.e., initial cavity length) and parallelism of the two surfaces that define the optically resonant cavity. The alignment of these surfaces during fabrication can represent one of the dominant factors in the cost of producing such a device.
One conventional fabrication method relies on the use of support structure that has multi-axis alignment capability. This support structure aligns and holds the multiple surfaces while adhesives are applied and cured to permanently fix them in their relative positions. Unfortunately, active alignment of the surfaces can be a time-consuming process. In addition, trapped air bubbles and internal stresses in the adhesives can lead to movement of the surfaces during and/or after the adhesives are cured.
Another conventional fabrication method relies on the monolithic integration of the surfaces. This typically entails the use of integrated circuit processing equipment in a semiconductor fabrication facility. Although such structures can exhibit exceptional alignment and parallelism, the costs associated with such equipment and facilities can be prohibitive.
Another conventional approach relies on forming alignment features, such as Vee grooves and trapezoidal holes, in each of the surfaces to be aligned. These alignment features are used to trap precision spacers, such as glass spheres or optical fibers, which determine the separation of the surfaces. Unfortunately, this approach has several drawbacks. First, the spacers can be very difficult to handle and insert into the alignment features. Second, the spacers must typically be fixed in the alignment features prior to assembling the multiple surfaces. As a result, minute volumes of an adhesive must be dispensed at each spacer location. Once the spacers are in place with the adhesive, a partial cure of the adhesive is performed to keep the spacers in place during the rest of the assembly process. Since the spacers are usually quite light, the adhesive tends to displace the spacers, at least slightly, from their respective alignment features. This results positional error. In addition, the need to form alignment features as well as the need to add an additional adhesive step increases the overall cost of this fabrication method.